1. Combustion lag is the time it takes after ignition for combustion to reach peak pressure. Normal combustion starts at the moment of ignition. Boyle’s law applies here as peak pressure develops and drops; the sudden drop in pressure helps cool the piston. Peak pressure does not happen at exactly top dead center, even though combustion happens before top dead center. Peak pressure usually happens at about 11 to 15 degrees after top dead center.
   1. Combustion is “building up pressure” from ignition to just past top dead center (TDC).
   2. “Push-of-war” between upward moving piston and earliest stage of combustion (from low pressure to peak pressure).
   3. Ignition timing before top dead center (BTDC) is necessary because it takes time to develop peak cylinder pressure.

Ideally, the peak occurs slightly after TDC for maximum piston push.